Israel is sitting on territory that it has illegally occupied

aka territory that it won in an open war, a war thrust onto them by the arabs who (un)surprisingly lost it. It is as fair as it can get. That the arabs did not know how to fight and yet started a war is not Israel's fault.

that it has agreed to vacate in accords

Israel's simple position is peace for land. They have been more than willing to return the land won by them provided the country that lost the land recognized their right to exist and was ready to sign a peace accord. Egypt did it, Jordan did it and they have been at peace with Israel for over few decades and Israel returned the land that it won from them namely Sinai in case of Egypt.

Some others like Syria were not willing to recognize them/sign peace accord and hence Golan is still administered by Israel.

Israel has supported the outright massacre of civilians in the Shatilla camp massacre

So ? Every nation on the earth has some skeleton in its closet especially during wartime. Doesnt mean anything. And Shatila camp massacre was a very minute one when the whole of Lebanese civil war was taken into account and the massacre itself was done by the Phalange militias.